Ligand Redox Noninnocence in Bis(phosphinimine) Diarylamido Pincer Complexes of Nickel

A series of Ni(II) complexes were generated through ligation by a chelating diarylamido ligand bearing diphenylmethylphosphinimine-based donors in the ortho positions of the aryl groups: HN[2-(NPPh₂Me)-4-R-C₆H₃]₂ (RH, Me). This ligand displays both chemical and redox noninnocence, including the generation of a rare, metal-bound aminyl radical. Several four- or five-coordinate Ni(II) complexes of various geometries were obtained, including chloride, phenoxide, and metallacyclic complexes, the latter of which result from ligand activation. Cyclic voltammetry (CV) of the four-coordinate complexes showed reversible oxidation events (E_1/2 = ca. –0.5 V vs. Fc^0/+) and the oxidation product of the phenoxide complex was fully characterized. The oxidation was determined to be primarily ligand-centered, as supported by electron paramagnetic resonance spectroscopy, X-ray crystallography, and density functional theory (DFT) calculations. More anodic, irreversible oxidation features were also observed for these complexes by CV (E_an = ca. +0.1 V vs. Fc^0/+). DFT computations on the second oxidation were consistent with a second ligand-centered process to generate nitrenium-like (R₂N⁺) electronic structures. These studies showcase the versatility of this pincer ligand both by stabilizing complexes of varying geometries and coordination numbers and by providing electrons during oxidation.